Removal of nitric oxide from gas mixtures containing the same



United States Patent This invention relates to the art of gaspurification, and is particularly concerned with the problem of removingnitric oxide from gas mixtures containing the same. In its specificapplication the invention consists in a process for removing nitricoxide present in low concentration in a coke oven gas or similar gasmixture containing gaseous oxidizable compounds.

This application is a division of my co-pending application Serial No.754,143, filed August 11, 1958, now Patent No. 3,023,076.

Nitric oxide and dienes (for example, cyclopentadiene) formpolymerizable nitrosates and nitrosites, which make trouble in city gas,gas meters, valves and pipe lines, or which as spontaneously flammableresins are the cause of explosions in the decomposition of coke oven gasby low-temperature cooling. Nitric oxide in fine gas hinders therecovery of carbon dioxide from flue gas by low-temperature washing,since it starts the decomposition of the washing agenttrichloroethylene. Nitric oxide in the recovered carbon dioxide makesthe latter unusable for application in the blasting of steel inconverters, because it remains in the steel as nitrogen. Nitric oxidewhich enters, from air contaminated with it, into the water circuit of ahydraulic washing plant, or which is formed by the action of bacteria inthe water circuit of the hydraulic washing plant, is transferred duringthe washing to the washed gas, for example, a NO-free converted gas, andjeopardizes the subsequent decomposition of the low-temperature gas.

For washing nitric oxide from gas mixtures, such asv coke oven gas, ithas been suggested to use aqueous solutions of salts of lower oxides ofsulphur or chromates and bichromatcs, respectively. However, theiraction is incomplete and they have therefore, not found application inthe industry. 7

The object of the present application is to provide a method ofoperation whereby the nitric oxide is removed completely andeconomically from mixed gases containing low concentrations of NO-gas,such as coke oven gas, generator gas and flue gas. It is characterizedin that the NO-containing gas mixture to be purified is washed with anaqueous solution of sodium chlorite, NaClO Solutions of chlorous acidHClO in water, to be used according to the invention, remove nitricoxide from the gas mixture conducted through it better than any otherwashing agent that has become known for the removal of NO. The best formof application of this solution is when it has a pH between 7 and 12,which condition can be easily standardized to the desired value by theaddition of OH-ions in the solution of technical sodium "icechlorite-particularly to the commercial form of the salt which has a lowcaustic soda content. In order to maintain this alkalinity, the sodiumchlorite washing stagein the purification of coke oven gas or convertedgas, which is to be decomposed by low-temperature cooling is shiftedbehind the working stages for removing the carbon dioxide: pressurewater washing and lye washing. For example, 10,000 Nmfi/h. coke ovengas, compressed to 12 atm. abs. and freed from carbon dioxide, whichstill contains 1 part per million NO, is washedin a wash tower filledwith Raschig ringswith 20 to 50 cubic meters per hour of a 1 to 10% (byweight) aqueous solution of NaClO at pH 9. NO can no longer be detectedin the washed gas. The washing agent is circulated by a circulation pumpbetween the base and head of the wash tower. Another Wash tower of thesame type as the first one serves to ensure the purity of the gas washedwhen the wash liquor of the first tower is almost exhausted. When thewashing power of the first tower has been spent, the second wash toweris connected as the first tower and the first one connected as thesecond tower, after replacing the washing agent by a fresh solution. Theexpression Nm. as used herein means one cubic meter of gas measured at 0C. and 760 mm. Hg.

If a gas mixture having a high carbon dioxide content, like flue gas,for example, which may contain 50* parts per million (p.p.m.) NO, is tobe liberated of nitric oxide for the eventual extraction of carbondioxide, it is washed with an aqueous acidified NaClO solution whose pHvalue is between 6 and 3, preferably 4.5, after which the washed gascontains no more NO. 7

The penetration of NO into gases subject to hydraulic washing for thepurpose of CO removal was discussed hereinbefore. According to theinvention the nitric oxide, penetrating the water, e.g., during aerationwith NO- containing air can be destroyed even in the water itself. Forthis purpose, the water is treated with NaClO until the dissolved NO hasvanished. The end point of the addition is visible in that the gaspassing through the water so treated does not yield NO any longer to agas conducted through it.

I claim:

1. A process for removing NO from a gas mixture References Cited in thefile of this patent UNITED STATES PATENTS Burdick May 11, 1920 OTHERREFERENCES Mellor: A Comprehensive Treatise on Inorganic and TheoreticalChemistry, Longmans, Green and Company, New York, N.Y., vol. 8, 1928,page 433.

1. A PROCESS FOR REMOVING NO FROM A GAS MIXTURE CONTAINING NO UP TOABOUT 50 PARTS PER MILLION OF NO WHICH CONSISTS IN WASHING THE GASMIXTURE WITH AN AQUEOUS SOLUTION OF NACIO2.